During initial start or cold start, the Power Stroke 7.3 Liter Diesel Engine high-pressure oil pump receives unfiltered oil from the left side valve lifter oil gallery through the anti-drain back check ball valve. Once the engine starts or during warm engine starts, the check ball closes and the high-pressure oil pump receives filtered oil from the high-pressure oil pump reservoir. The high-pressure oil pump pumps the oil under extremely high pressures 4,115-20,577 kPa (600-3,000 psi) through the left and right side high-pressure supply hoses to the high-pressure oil rails (integral to the cylinder heads). Once in the oil rail, the oil is fed to the fuel injector bores through four oil feed galleries drilled and machined in the cylinder head. The high-pressure oil then actuates the fuel injectors.